During drilling of oil and gas wells, it is desirable to test earth formations to determine their productive characteristics, e.g. how much oil and/or gas may be in the formation and how fast it can be produced. It is desirable to learn this information as soon as possible, e.g. before decisions are made to spend the money needed to complete a well for permanent production. One type of testing before completion is referred to as drillstem testing, since the primary work string available during drilling is the drillstring itself, although the equivalent testing may be done with other work strings or with a wireline supported tool.
One conventional drillstem test allows fluids produced from the formation to flow up the drillstring for a period of time. The drillstring is typically provided with a packer that is set in the annulus between the drillstem and the borehole above the formation of interest. A valve in the drillstring may then be closed shutting in the well so that the pressure below the packer may stabilize at natural formation pressure. The test equipment normally includes pressure and temperature sensors to measure and record and/or transmit to the surface bottomhole pressure data and temperature data. After the downhole conditions have stabilized, the valve in the drillstring is opened allowing formation fluids to flow up the drillstring while downhole pressure and temperature are measured. After a quantity of fluids is produced, the valve is usually closed again and pressure and temperature are measured as the downhole pressure returns to its natural formation pressure. Various characteristics of the formation may be derived from the produced fluids and from the pressure and temperature data collected.
The conventional open flow drillstem tests often result in production of large quantities of hydrocarbons when facilities have not yet been installed for handling such quantities. To avoid this and other problems, the closed-chamber drillstem test was developed. In closed-chamber testing, a portion of a drillstring or other tubing is provided with a pair of valves allowing flow through the tubing to be controlled at two spaced apart locations in the tubing. The space in the tubing between the valves forms a test chamber. A packer is typically used to seal the annulus above the formation to be tested and the lower valve is closed to allow pressure in the borehole below the packer to stabilize at natural formation pressure. Pressure and temperature sensors monitor conditions in the borehole. While the lower valve is closed, the test chamber is initially filled, at least partly, with a gas and the upper valve is closed. Some liquid may also be placed in the chamber, but the pressure in the chamber at the lower valve is set below the natural formation pressure. After borehole conditions stabilize, the lower valve is opened allowing formation pressure to flow formation fluids into the test chamber compressing the gas in the test chamber. Flow reduces as chamber pressure increases and stops when the pressure at the bottom of the test chamber reaches the natural formation pressure. Pressure and temperature data is recorded as the test is performed. In a properly designed closed-chamber drillstem test, the data covers a continuous range of flow rates extending from an initial high value to essentially no flow at the end of the test. Data from such a properly designed test may be analyzed by known methods to determine the formation characteristics. The closed-chamber test results in less produced fluids that need to be disposed of, may take less time than open flow testing, and has other advantages. However, if the closed-chamber system is not properly designed, the chamber may fill too quickly, resulting in insufficient data for good analysis, or too slowly, resulting in either an incomplete test if it is terminated too soon or an undesirably long test period.